Camshaft phasers for varying the phase relationship between the crankshaft and a camshaft of an internal combustion engine are well known. In a typical vane-type cam phaser, a controllably variable locking pin is slidingly disposed in a bore in a rotor vane to permit rotational locking of the rotor to the sprocket, and hence to the stator, under certain conditions of operation of the phaser and engine. A known locking pin mechanism includes a return spring to urge an end of the pin into a hardened seat disposed in the pulley or sprocket (pulley/sprocket) of the phaser, thus locking the rotor with respect to the stator. The rotor may be formed of aluminum, and a steel bushing is pressed and staked into the bore at a predetermined axial location to guide the pin. In at least one prior art embodiment, the pin is shouldered, which shoulder engages the rotor bushing as a limit stop to pin travel. In operation, the pin is forced from the bushing and well in the pulley/sprocket to unlock the rotor from the stator by pressurized oil supplied from a control valve in response to a programmed engine control module (ECM).
A prior art phaser has at least two shortcomings that are overcome by an improved phaser in accordance with the invention.
First, the pin and the seat typically include mating annular bevels to center the pin in the seat and thereby minimize angular lash between the rotor and the sprocket while locked. If the pin is permitted to engage the seat fully, however, the pin may become jammed into the seat and not respond reliably to opening oil pressure. Therefore, a shoulder is provided on the pin to limit travel thereof. It is known that, with repeated use, the pin shoulder can displace the rotor bushing axially, resulting in erratic operation of the locking pin mechanism.
Second, when it is desired to engage the pin to lock the rotor to the sprocket, oil pressure is withheld from the pin end axial face in the well, allowing the spring force to eventually (in milliseconds) overcome the force exerted on the pin end face by the diminishing oil pressure. The force required is proportional to the surface area of the end of the pin. A rapid locking response is benefited by a relatively strong spring (high spring rate); however, in the reverse situation, that of unlocking the pin, a high rate spring results in a relatively slow unlocking response. Hydraulic unlocking force on the pin end is constant but spring resistance increases as the spring is progressively compressed. Thus, the pin initially assumes a relatively high linear velocity which then may slow significantly before the pin is fully withdrawn from the sprocket, resulting in a relatively slow response overall.
What is needed is a means for increasing the withdrawal rate of the locking pin during unlocking of the rotor from the stator/sprocket.
It is a principal object of the present invention to increase the speed of response of a vane-type camshaft phaser in unlocking a rotor from a stator/sprocket.
It is a further object of the invention to increase the locking stability of a rotor-locking mechanism in a vane-type camshaft phaser.